1. Field of the Invention
The present invention relates to an image forming apparatus which forms a color image by superposing images of a plurality of colors formed by a plurality of print units.
2. Description of Prior Art
In a color image forming apparatus of tandem structure, an image of four colors is formed at the same time in a scan. In the color image forming apparatus, four print units including photoconductors for the four print colors are aligned, and images of the four colors are supplied to exposure positions for each color, by delaying in correspondence to the distances between the photoconductors. In a system wherein four beams are scanned with a polygon mirror, the direction of luster scan becomes reverse to each other between the former two colors and the latter two colors, and the data are corrected therefor. Further, color shifts are caused by various factors.
Print positions for the four colors may be shifted due to image delay by the memories and the shift of exposure positions. Then, color displacement occurs, and image is distorted. When such image distortion is prevented, an image of high quality can be obtained. Then, in order to correct the shift of print positions, it is proposed to detect image distortion quantities and to change the line address in the main scan direction for the colors for an address in the main scan direction according to the detection results.
However, the detection of the distortion quantities for distortion correction is complicated when the line address in the main scan direction for the colors for an address in the main scan direction is changed according to the detection results of image distortion. Further, in order to detect the image distortion quantity correctly, sensors therefor have to be set correctly, and it takes a long time to set the sensor.